This invention relates to the preparation of sulphides and selenides, including related compounds, for example thiogallates. The invention has application in the synthesis of phosphors, pigments, ceramics and optoelectronic materials. The phosphors may be used for photoluminescence, high and low voltage cathodoluminescence, as well as for AC or DC electroluminescence. The phosphors can also be used in x-ray applications and for thermoluminescent storage.
Known methods of preparing sulphides and selenides are:
i) the reaction between gaseous hydrogen sulphide or hydrogen selenide and a solution containing the appropriate cation, and
ii) the thermal and/or catalytic decomposition of an organo-sulphur or organo-selenium compound in a solution containing the desired cation.
Typical of the reagents employed in the second method are thiourea, thioacetamide and selenourea. This method employs controlled release of the anionic species uniformly throughout the reaction container and is commonly known as homogeneous precipitation.
Both methods have some disadvantageous features. In the first method the use of highly toxic gaseous reagents can be extremely hazardous. In addition, it is believed that this method leads to the precipitation of non-stoichiometric materials having an irregular, plate-like morphology. As to the second method, the principal disadvantage of most organo-sulphur or organo-selenium compounds is their high cost. A further disadvantage is that the precipitated product is usually contaminated with the organic starting compound. It is an object of the invention to provide a method of preparing sulphides and selenides in which the above disadvantages are mitigated.
According to the invention a method of preparing sulphides and selenides comprises the steps of dissolving sulphur or selenium in hydrazine hydrate and then combining the resulting solution with a solution of an appropriate cation to precipitate the corresponding sulphide or selenide.
In some embodiments the hydrazine hydrate is used in an aqueous solution.
It is known that sulphur, but not selenium, shows solubility in anhydrous hydrazine. However the use of this compound as a solvent is not really practical as it is well-known that it is highly reactive. It has also been reported that a slow reaction takes place when sulphur is mixed with hydrazine hydrate. Such reports lead to the expectation that hydrazine hydrate is not of practical use as a quantitative solvent for these elements. Unexpectedly it has now been found that both sulphur and selenium dissolve in hydrazine hydrate sufficiently rapidly to enable binary and ternary compounds of these elements to be prepared in quantity.
An important feature of the method described herein is the dissolution of elemental sulphur or selenium, which can be of a high purity, by chemical reaction with hydrazine. This may be expressed as:
N2H4+2X=N2+2H2X
(where X=S or Se)
The hydrazine is preferably in form of hydrazine hydrate or a solution thereof.
In order to yield stable and usable solutions an excess of hydrazine hydrate with an approximately 2:1 mole ratio or greater of hydrazine hydrate to X is desirable. The solution can subsequently be diluted with de-gassed water or a suitable organic solvent such as N,N-dimethyl-formamide or dimethylsulphoxide. If maintained in a sealed container solutions produced in this way can be stored for a long time. The simple mixing of such a sulphur or selenium bearing solution with a solution of the desired cation leads to the rapid precipitation of the respective sulphide or selenide.
The precipitation technique lends itself to the preparation of amorphous or crystalline sulphides, depending on the conditions of precipitation and subsequent heat treatments. Thus, smaller particles can be produced by precipitation in organic solvents such as methyl alcohol, ethyl alcohol, N,N-dimethyl-formamide or dimethylsulphoxide.
In general, sulphides can be precipitated from aqueous solutions. Aqueous solutions can be used in the formation of sulphides of elements of Groups IIB, IIIA, IVA and VA of the periodic table from which phosphors of chemical formulas such as ZnS:Mn, ZnS:Ag and ternary compounds of formulas such as ZnxCd1xe2x88x92xS:Ag (where 1xe2x89xa7xxe2x89xa70), ZnS:Cu and ZnS:Mn,Cu may be synthesised. Sulphides can also be precipitated from aqueous solutions of compounds containing the transition elements. Transition elements are defined herein as elements which have partly filled xe2x80x98dxe2x80x99 or xe2x80x98fxe2x80x99 shells including elements that have partly filled xe2x80x98dxe2x80x99 or xe2x80x98fxe2x80x99 shells in any of their oxidation states. Compounds of elements of the following sub-groups, namely: gallium, indium and thallium; germanium, tin and lead; and arsenic, antimony and bismuth; can also be prepared. Furthermore the invention is not limited to the use of aqueous solutions and non-aqueous solutions of hydrazine hydrate can also be used.
It is also possible to produce precursors for Group IIA sulphides and selenides and ternary sulphides and selenides (such as thiogallates, seleno-gallates and sulpho-seleno-gallates) from aqueous solutions by treating with suitable alkalis or alkaline carbonates such as ammonium carbonate. From such solutions compounds such as barium zinc sulphide and a range of thiogallates, for example zinc thiogallate and strontium thiogallate, can be prepared. These compounds can be suitably mixed with activator elements or compounds such as manganese, bismuth or copper for example or with rare earth elements such as europium or cerium, during the precipitation stage or later. These compounds or mixtures may then be fired at a higher temperature in suitable atmospheres, for example sulphur, nitrogen argon or air, to produce efficient phosphors. Thus phosphors having formulas such as Ba2ZnS3:Mn, ZnGa2S4:Mn and SrGa2S4:Mn can be produced by this method.
Selenides of the elements referred to above can similarly be precipitated from solutions containing the appropriate cations. Selenides that are particularly sensitive to oxidation may be precipitated in the presence of an inert or aprotic non-aqueous solvent such as N,N-dimethyl-formamide. A particular example of this requirement is found in the precipitation of zinc selenide. In this case, a buff-coloured precipitate of solvated zinc selenide is initially formed. The solvent is removed by heating the precipitate in an inert atmosphere to a temperature in the range 200-400xc2x0 C. The heating may be carried out under reduced pressure.
The selenides of the other elements can also be produced in this way. In addition, ternary compounds such as compounds of formula ZnSxSe1xe2x88x92x may be precipitated by employing a single solution of hydrazine hydrate containing both dissolved sulphur and dissolved selenium. It is also possible to precipitate ternary compounds of formula such as CuInSe2 or CuInS2 by including copper and indium cations in the solution that is mixed with the hydrazine hydrate solution.
Methods embodying the invention can readily be used to produce not only phosphors but also a range of photoconducting materials such as doped cadmium sulphide. In addition these methods can also be used to produce sulphide and selenide based pigments from a range of metal solutions containing copper, silver or iron for example.
The advantages of the methods described herein over alternative methods are:
i) the preparation of high purity sulphides and selenides and mixtures thereof,
ii) low production costs in comparison with methods using organic thio or seleno reagents,
iii) avoidance of the enormous costs associated with the removal of hydrogen sulphide or selenium sulphide.